The present invention relates to spread-spectrum communication systems, and more particularly to a multipath-combining subsystem and method for use with a spread-spectrum receiver which can receive a spread-spectrum signal arriving from a plurality of paths.
In spread-spectrum environments, multipath presents a problem in terms of synchronization and signal reliability. Typically, a spread-spectrum signal is transmitted from a transmitter and, as shown in FIG. 1, can be reflected from a number of surfaces, such as buildings, mountains, trees, trucks, etc. In the microwave region, the multipath problem is more acute, due to the propagation characteristics of microwaves.
A RAKE system can be used for selecting the strongest path in a multipath system. Such a system is described in U.S. Pat. No. 5,081,643. When using matched filters, a different problem arises. For each path of a multipath signal, a match of the chip sequence within the spread-spectrum signal may occur within the matched filter, producing an output. While the strongest output might be chosen, the subsequent outputs may have power levels close to the strongest output or at least sufficiently strong to cause false triggers and other problems.
Multipath can be aggravated by the time varying nature of propagation characteristics due to motion of a vehicle, a receiver, a transmitter, and objects from which reflections may occur. Thus, while the strong path may be locked onto at one point in time, the strong path may move in time to a different path due to the multipath environment.
A general object of the invention is a subsystem and method for receiving a spread-spectrum signal arriving at different times from a plurality of paths.
Another object of the invention is a subsystem and method which, in place of selecting the strongest signal, instead enhances reception of a multipath signal by combining power from the various paths.
According to the present invention, as embodied and broadly described herein, a multipath-combining subsystem and method for use with a spread-spectrum receiver for receiving a spread-spectrum signal is provided. The spread-spectrum signal is assumed to be arriving at different times, from a plurality of paths. The plurality of paths typically might be due to reflections of the spread-spectrum signal off of buildings, automobiles, and other objects which may be found in the environment.
The spread-spectrum signal has a plurality of packets, with each packet having a header followed by a data portion. The header includes a header-chip-sequence signal. The data portion includes a data-symbol-sequence signal, with each data symbol of the data-symbol-sequence signal spread-spectrum processed by a data-chip-sequence signal.
The multi path-combining subsystem includes matched-filter means, a header memory, and a combiner. The matched-filter means may be embodied as a header-matched filter in parallel with a data-matched filter, or alternatively, as a programmable-matched filter. For the first embodiment, the header-matched filter has a first impulse response matched to the header-chip-sequence signal, and the data-matched filter has a second impulse response matched to the data-chip-sequence signal. For the second embodiment, the programmable-matched filter initially has the first impulse response matched to the header-chip-sequence signal, and subsequently has the second impulse response matched to the data-chip-sequence signal.
The following discussion uses, by way of example, the header-matched filter and data-matched filter. The programmable-matched filter can accomplish the same function as the header-matched filter and the data-matched filter, by having the impulse response of the programmable-matched filter change from the first impulse response to the second impulse response, thereby matching the header-chip-sequence signal and the data-chip-sequence signal, respectively.
The header-matched filter detects, within a packet and for each path of the spread-spectrum signal, each match of the header-chip-sequence signal with the first impulse response. The detection is compared to a threshold, denoted herein as the header threshold, and when the header threshold is met, the header-matched filter outputs a header-detection signal. The header-detection signal has a header amplitude, and a respective chip location with respect to the header-chip-sequence signal.
The time difference between the receipt of each path of the spread-spectrum signal is assumed to be greater than the time of each chip of the header-chip-sequence signal, and greater than the time of each chip of the data-chip-sequence signal.
The header memory stores the header amplitude of each header-detection signal and the respective chip location of each header-detection signal.
The data-matched filter detects, at each respective chip location of each header-detection signal for each path, each match of the data-chip-sequence signal with the second impulse response. The data-matched filter outputs, in response to each detected match, a data-detection signal. Each data-detection signal has a data amplitude.
The combiner, which typically includes a memory and adder gates, multiplies the header amplitude of each header-detection signal by the data amplitude of each data-detection signal, at each corresponding chip location, respectively. The multiplication generates a plurality of weighted elements for each data symbol. The combiner then adds the plurality of weighted elements for each data symbol to generate a sum signal. The sum signal typically is detected by a detector and outputted as data.
Additional objects and advantages of the invention are set forth in part in the description which follows, and in part are obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention also may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims.